Grippers for gripper conveyors intended for the separate transportation of objects, such as printed matter, have, in practice, earlier been of complex construction and, among other things, have included a releasable latching device by means of which the gripper is held latched in a closed state. When the gripper passes a stationary cam, the latch is released and the gripper opened so as to allow the object to fall from the gripper. Such latch equipped grippers have the advantage that the gripper will remain open after having released the object, and that, in conjunction with gripping a further object, the gripper need only be closed around this object and then latched or locked automatically in its closed state. Furthermore, the known grippers have the advantage that a newspaper or like printed matter is readily afforded the time needed to drop from a downwardly facing gripper whose release latch has been activated, irrespective of whether neighbouring grippers have been opened or not.
However, one troublesome drawback with such latch equipped grippers resides in their complexity and therewith in their expense and sensitivity to disturbances. These known grippers also incur high service costs.
Consequently, in practice, the trend has been towards the use of self-closing grippers, i.e. grippers which include two legs which can be pivoted in relation to one another and which are biased towards one another by means of a spring device. Such self-closing grippers are based on the clothespeg principle and afford important advantages, in that the grippers can be produced at low costs, given a simpler function, are highly reliable in operation and incur low service costs. These known self-closing grippers are constructed to be opened temporarily as they pass and are activated by a stationary gripper opening cam. In this regard, a problem is encountered when desiring to open solely one or a few grippers in order to allow the grippers to release the object held thereby, i.e. a newspaper or like object. The problem occurs when it is desired to extract one or a few newspapers or like printed matter from the newspaper stream in order, for instance, to check the quality of the print, without disturbing the smoothness of the newspaper stream in the continued handling thereof. A similar problem also arises when desiring to extract single newspapers from the stream, for instance when wishing to open e.g. each alternate gripper or every third gripper in order to form a newspaper bundle, an overlapping stream of newspapers, or the like, and further difficulties arise when wishing to switch between different desired gripper opening programs.
The problem indicated arises from the following circumstances:
The gripper spacing on the conveyor is relatively small, for instance 50 mm (or about 2 inches), while the conveyor is moved at a relatively high speed, e.g. a speed of 1 m/s, so that the conveyor will have the desired high transport capacity, e.g. an hourly capacity of 72,000 newspapers.
Furthermore, for safety reasons, the gripping depth of the grippers (the leg length) must fulfill a given minimum value, for instance a value of 45 mm. The time taken for a newspaper to fall gravitationally from a downwardly facing, open gripper pass free from the extremities of the gripper legs, will thus depend on the leg gripping depth. In the case of those transporting situations that are now often relevant and that have been described by way of example above, the time interval between two immediately adjacent grippers on the moving conveyor is shorter than the time required for the newspaper to fall free from the gripper. It will be seen from this that the conventional technique of opening such single grippers, i.e. the use of a stationary gripper opening cam, is encumbered with certain deficiencies, since if the cam is made sufficiently long to activate a gripper over the time period required for the newspaper to have time to fall from the gripper, the cam will also activate the next-following gripper, so that the position of the newspaper held by this gripper is displaced before the newspaper is again gripped by the gripper when the cam is lifted to one side. Because the position of the newspaper in the gripper last activated is disturbed, problems occur further on in the handling of the product series, and particularly when this series has the form of an overlapping newspaper stream.
Consequently, in accordance with the known technique, it has been necessary in conjunction with the use of self-closing grippers either to restrict the conveying capacity, so as to ensure that individual grippers will be opened in a functionally correct fashion and thus enable correct release of individual newspapers, or it has been necessary to accept limited usefulness of self-closing grippers, namely when all grippers shall be opened at a station with the aid of a stationary gripper opening cam of sufficient height.
CH-A-667 860 reveals a conveyor with self-closing grippers, of which only every second or all grippers are temporarily opened at an opening station, by means of an orbiting series of opening cams. US-A-3,713,648 reveals a conveyor in which all grippers are temporarily opened at an opening station by means of a circulating series of opening cams.